Vehicles today use, for example, urea as reductant in SCR (selective catalytic reduction) systems which comprise an SCR catalyst in which said reductant and NOx gas can react and be converted to nitrogen gas and water. Various types of reductants may be used in SCR systems. AdBlue is an example of a commonly used reductant.
One type of SCR system comprises a container which holds a reductant. The SCR system has also a pump adapted to drawing said reductant from the container via a suction hose and to supplying it via a pressure hose to a dosing unit situated adjacent to an exhaust system of the vehicle, e.g. adjacent to an exhaust pipe of the exhaust system. The dosing unit is adapted to injecting a necessary amount of reductant into the exhaust pipe upstream of the SCR catalyst according to operating routines which are stored in a control unit of the vehicle. To make it easier to regulate the pressure when there are small or no dosing amounts, the system comprises also a return hose which runs back to the container from a pressure side of the system. This configuration makes it possible to cool the dosing unit by means of the reductant which, during cooling, flows from the container via the pump and the dosing unit and back to the container. The dosing unit is thus provided with active cooling. The return flow from the dosing unit to the container may be substantially constant and is currently not controlled or regulated by means of appropriate valves or such units.
In certain conditions, air may enter the SCR system upstream of the pump. This may for example occur during start-up of the SCR system after its initial fitting, in which case there will be air in the suction hose.
Air may also enter the suction hose when the SCR system has used up all the available reductant in the container, in which case the container will be empty of reductant, the pump runs dry and air is drawn into it via the suction hose.
Another example is that air may enter the suction hose in situations where there is a limited amount of reductant left in the container in the SCR system and said SCR system moves in such a way that splashing occurs in the container, in which case air may be drawn into the pump via the suction hose.
A further example is that the suction hose may be incorrectly fitted to the pump such that air leakage occurs on the upstream side of the pump. Here again air may be drawn into the pump via the suction hose or at a faulty or damaged seal between the suction hose and the pump.
An example is that the suction hose may itself be frayed or defective in such a way as to allow air to be drawn into the pump via the hose.
Any air entering the pump on an inlet side of it will adversely affect the reductant flow in the SCR system, thereby reducing a cooling power of the dosing unit, with potential risk of overheating of temperature-sensitive components of the dosing unit.
Emissions of the SCR system may also be adversely affected by presence of air at the pump in that the reductant supply to the dosing unit will be limited.
Any presence of air at the pump in the SCR system adversely affects a working pressure of the dosing unit. Building up a normal working pressure of the SCR system also currently takes quite a long time when there is air in the pump.
DE 102008030756 A1 refers to detection of air downstream of a pump for supplying a dosing unit with reducing agent in an SCR system and for clearing gas bubbles etc. from a line between the pump and the dosing unit by means of reducing agent via a valve and a return line for leading reducing agent back to a reducing agent tank.
There is thus a need to improve current SCR systems in order to reduce or eliminate the above disadvantages.